The present invention relates to marking semiconductor wafers such as silicon wafers for subsequently identifying the wafers during integrated circuit fabrication. In particular, the present invention includes a novel binary dot-array encoding scheme for forming reliable, machine-readable identifying markings on a wafer.
It is known in the integrated circuit industry to mark a semiconductor wafer, such as a silicon wafer, early in the integrated circuit fabrication process with an identifying number for identifying the wafer during subsequent processing. The identifying number typically consists of an ordered series of digits that indicate, for example, a fab facility number, lot number and wafer number or the like. The wafers typically are tracked on a computer system to control inventory or to select or control one or more processing steps depending upon the identity a particular wafer.
One known method of marking wafers for such purposes is the use of a laser scribe system. Such a system includes a laser source which "writes" on a wafer in a predetermined pattern, for example to form a numeral or series of numerals, as further described below. The laser beam melts and vaporizes the silicon to form a trench or depression in a predetermined location. Alphanumeric characters may be formed in that manner.
Typically, characters written by a laser scribe system measure in a range of from about 16 by 32 mils up to three times that area. It has been found that such characters may be read by the naked eye under very good light conditions. The characters are difficult to read, however, because silicon wafers are very shiny, i.e. reflective, and the characters are the same color as the remainder of the wafer, so contrast is very limited. Larger characters are easier to discern, but the size of the characters must be traded off against sacrificed wafer area, as circuits formed in the same area as the laser markings are likely to be defective for that reason. Smaller, yet reliably readable identifying markings would be desirable.
A vision system can be used in conjunction with a laser scribe system to allow a computer system to read a wafer identifying number. A vision system allows for greater automation of the integrated circuit fabrication process. It generally incorporates an optical input device such as a microscope or camera, coupled to a light receiver such as a CCD array, in turn coupled to a computer for processing the raw data in an effort to recognize an alphanumeric character.
Automated alphanumeric character recognition is very difficult in the context of silicon wafers. As noted, reflected light can obscure the image, and contrast is very low. Additionally, wafer processing steps such as doping, applying films, photoresist, etc. degrade the laser markings by partially filling in the depressions formed by the laser beam. Consequently, the vision system input data is inherently unreliable. Obviously, misread or unreadable wafer identifying numbers leads to added handling, defective parts, or loss of time and/or materials, all of which translate to higher circuit fabrication costs. Use of redundancy of identifying markings, for example voting on three copies of the number, would improve reliability, but at substantial wafer area expense. Check sums can be added, but again at an expense of two additional characters.
Vision systems adequate for character recognition are expensive, presently costing on the order of $30,000 for hardware. Additional expense is incurred for character recognition software. What is needed is to improve the reliability of reading wafer identifying data, at lower cost, and while minimizing sacrificed wafer area.